1. Field of the Invention
The present invention relates to an active vibratory noise control apparatus for canceling vibratory noise which is produced in the passenger compartment of a vehicle by a variable-cylinder internal combustion engine that can selectively be operated in a full-cylinder operation mode in which all of the cylinders are operated and a partial-cylinder operation mode in which some of the cylinders are out of operation.
2. Description of the Related Art
Heretofore, there has been proposed a method of actively controlling vibratory noise caused in the passenger compartment of a vehicle by the internal combustion engine mounted on the vehicle, and there has also been put to practical use an active vibratory noise control apparatus. The active vibratory noise control apparatus which controls vibratory noise in the passenger compartment uses a signal representative of the rotational speed of the output shaft of the internal combustion engine, which is highly correlated to the vibratory noise to be controlled, or vibrations having a frequency based on suspension vibrations, as a basic signal. A canceling signal for canceling the vibratory noise in the passenger compartment is generated by an adaptive filter based on the basic signal. The canceling signal is converted from a digital signal into an analog signal, filtered by a low-pass filter, amplified by an amplifier, and applied to a speaker in the passenger compartment, which converts the canceling signal into a reproduced sound.
A reference signal generating circuit corrects the basic signal based on corrective data depending on the signal transfer characteristics in the passenger compartment between the speaker and a microphone located in the passenger compartment, generating a reference signal. The microphone detects an error signal based on the vibratory noise in the passenger compartment. An LMS algorithm processing circuit calculates filter coefficients of the adaptive filter based on the reference signal and the error signal in order to minimize the error signal, and sequentially updates the filter coefficients in an adaptive feed-forward control process for canceling the vibratory noise in the passenger compartment.
However, since the above active vibratory noise control apparatus performs the adaptive feed-forward control process based on the signal transfer characteristics between the speaker and the microphone, if the signal transfer characteristics are brought out of preset conditions as when a window of the vehicle is opened, then the signal transfer characteristics change, causing the active vibratory noise control apparatus to malfunction.
In an attempt to solve the above problem, there has been developed an active vibratory noise control apparatus which stops its vibratory noise control process by detecting when the signal transfer characteristics are brought out of preset conditions as when a window of the vehicle is opened (e.g., see Japanese laid-open patent publication No. 6-295187).
For reducing muffled sounds due to the vibratory noise caused by the internal combustion engine, the active vibratory noise control apparatus detects the frequency of the vibratory noise to be controlled from the rotational speed of the internal combustion engine, and generates a basic signal having a frequency based on the detected frequency.
There has been known a variable-cylinder internal combustion engine which changes the number of active cylinders depending on the operating state of the engine for improving fuel economy (e.g., see Japanese laid-open patent publication No. 61-212638).
If the above active vibratory noise control apparatus is incorporated in a vehicle powered by a variable-cylinder internal combustion engine, then since the engine produces different vibration frequencies (rotational order components to be controlled) due to fuel combustion upon rotation of the output shaft of the internal combustion engine in a full-cylinder operation mode and a partial-cylinder operation mode, the active vibratory noise control apparatus allows unexpected vibratory noise, such as annoying sounds, to be produced when the active vibratory noise control apparatus that is set to reduce vibratory noise in the passenger compartment in one of the above modes operates in the other mode because it tends to become unstable in the other mode.
The active vibratory noise control apparatus that stops the vibratory noise control process by detecting when the signal transfer characteristics are brought out of preset conditions is only designed to solve problems which arise when the signal transfer characteristics in the passenger compartment to be controlled change. The active vibratory noise control apparatus is unable to solve problems which are caused by switching between different vibration sources such as the full-cylinder operation mode and the partial-cylinder operation mode.